Steels have been used by mankind for at least 3,000 years and are widely utilized in industry comprising over 80% by weight of all metallic alloys in industrial use. Existing steel technology is based on manipulating the eutectoid transformation. The first step is to heat up the alloy into the single phase region (austenite) and then cool or quench the steel at various cooling rates to form multiphase structures which are often combinations of ferrite, austenite, and cementite. Depending on steel compositions and thermal processing, a wide variety of characteristic microstructures (i.e. polygonal ferrite, pearlite, bainite, austenite and martensite) can be obtained with a wide range of properties. This manipulation of the eutectoid transformation has resulted in the wide variety of steels available nowadays.
Currently, there are over 25,000 worldwide equivalents in 51 different ferrous alloy metal groups. For steel produced in sheet form, broad classifications may be employed based on tensile strength characteristics. Low-Strength Steels (LSS) may be defined as exhibiting ultimate tensile strengths less than 270 MPa and include types such as interstitial free and mild steels. High-Strength Steels (HSS) may be steel defined as exhibiting ultimate tensile strengths from 270 to 700 MPa and include types such as high strength low alloy, high strength interstitial free and bake hardenable steels. Advanced High-Strength Steels (AHSS) steels may have ultimate tensile strengths greater than 700 MPa and include types such as martensitic steels (MS), dual phase (DP) steels, transformation induced plasticity (TRIP) steels, complex phase (CP) steels and twin induced plasticity (TWIP) steels. As the strength level increases, the ductility of the steel generally decreases. For example, LSS, HSS and AHSS may indicate tensile elongations at levels of 25% to 55%, 10% to 45% and 4% to 50%, respectively.
AHSS have been developed for automotive applications. See, e.g., U.S. Pat. Nos. 8,257,512 and 8,419,869. These steels are characterized by improved formability and crash-worthiness compared to conventional steel grades. Current AHSS are produced in processes involving thermo-mechanical processing followed by controlled cooling. To achieve the desired final microstructures in either uncoated or coated automotive products requires a control of a large number of variable parameters with respect to alloy composition and processing conditions.
Further developments of AHSS steels, designed for specific applications, will require careful control of alloying, microstructure and thermo-mechanical processing routes to optimize the specific strengthening and plasticity mechanisms responsible, respectively, for the desirable final strength and ductility characteristics.